Downgrade outgoing metadata to server's schema version on upload

[yarikoptic]

Summary

When the DANDI Archive server reports an older schema_version than the one
the client's dandischema is generating (as happened with
dandi/dandi-schema#342 before it was released as
dandischema==0.14.0), uploads fail because the server rejects metadata
with unknown fields. This PR wires an on-the-fly downgrade into every
call site where a metadata dict leaves the client:

DandiAPIClient._maybe_downgrade_metadata(metadata) is a no-op unless
the server is strictly older and its schema version is in
dandischema.consts.ALLOWED_TARGET_SCHEMAS; otherwise it delegates to
dandischema.metadata.migrate(..., to_version=<server_ver>, skip_validation=True).
check_schema_version's warning now tells the user whether an automatic
downgrade will be attempted or whether the library cannot downgrade to
this version at all.

Also updates the dandischema constraint to
>= 0.12.0, != 0.13.0, < 0.15.0. 0.13.0 is excluded because it dropped
the SIMPLE_DOWNGRADES entries for releaseNotes and sameAs, so
uploads to older Archive servers silently regress (the fall-through
short-circuits and sends unchanged metadata that the server rejects).
0.14.0 restored them.

Tracks: dandi/dandi-schema#343 · Depends on: dandi/dandi-schema#342 (released as dandischema==0.14.0)

Downgrade dispatch (single interceptor)

Rather than sprinkle _maybe_downgrade_metadata(...) at every metadata-
sending call site, DandiAPIClient.request runs
_maybe_downgrade_request_metadata over the outgoing json body.
A body is considered to carry DANDI metadata iff it is a dict whose
"metadata" value is itself a dict with both schemaKey and
schemaVersion as strings — the invariant shape of every DANDI
metadata payload the archive accepts. Non-matching bodies pass
through unchanged.

Any future metadata-sending endpoint gets the downgrade for free —
no per-call-site plumbing required.

Behavior when downgrade can't strip a populated field

dandischema.metadata.migrate raises ValueError if a
post-server-version field (sameAs added 0.8.0, releaseNotes added
0.7.0) carries a non-empty value that can't be simply stripped. The
client catches this, logs a warning, and forwards the original
metadata so the server decides. Prior draft of this PR let the
ValueError propagate and crash the upload.

Graceful behavior against older installed dandischema

• With dandischema==0.12.0 (client itself generates 0.7.0 metadata,
  matching what old servers speak) — _maybe_downgrade_metadata
  short-circuits: nothing to downgrade from. Same behavior as before
  this PR.
• With dandischema==0.13.0 — the client generates 0.8.0 metadata but
  the library has no downgrade path. This combination breaks uploads
  to older Archive servers, which is why 0.13.0 is excluded from the
  version range.
• With dandischema>=0.14.0 — full downgrade path is active.
• The new tests that assert downgrade behavior are gated via a
  _needs_downgrade skip marker that requires both
  DANDI_SCHEMA_VERSION >= 0.8.0 and "0.7.0" in ALLOWED_TARGET_SCHEMAS,
  so py3-lowest (which resolves to dandischema==0.12.0) cleanly
  skips them while the default env exercises them fully.

Test plan

• pytest -k "downgrade or check_schema_version or retry_logging"
  against dandischema==0.14.0 (default env) — 23 passed
• Same, under tox -e py3-lowest with dandischema==0.12.0 — 15
  passed, 8 skipped cleanly via the tightened _needs_downgrade gate
• tox -e py3-lowest -- dandi/tests/test_metadata.py dandi/tests/test_utils.py dandi/tests/test_files.py dandi/tests/test_helpers.py dandi/tests/test_organize.py dandi/validate/tests/ — 1023 passed,
  302 skipped, 16 xpassed
• tox -e lint and tox -e typing clean
• Full CI green
• Reviewer: sanity-check _maybe_downgrade_metadata fall-through
  wording in the check_schema_version warning (msg_downgrade) —
  it now branches on whether the library supports downgrade to the
  target version

[codecov]

Codecov Report

❌ Patch coverage is 90.52632% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.01%. Comparing base (2b5f8ea) to head (85efc16).

Files with missing lines	Patch %	Lines
dandi/dandiapi.py	77.50%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1898      +/-   ##
==========================================
+ Coverage   76.96%   77.01%   +0.05%     
==========================================
  Files          88       88              
  Lines       12882    12957      +75     
==========================================
+ Hits         9914     9979      +65     
- Misses       2968     2978      +10     

Flag	Coverage Δ
unittests	77.01% <90.52%> (+0.05%)	⬆️

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